This invention relates to high frequency gas discharge lamp dimming ballasts, and more particularly relates to a novel control circuit for such devices which enables accurate dimming control at low light output levels. This application is a continuation-in-part of copending application Serial No. 642,072 filed Aug. 17, 1984 entitled "High Frequency Gas Discharge Lamp Dimming Ballast" in the names of David Luchaco and Dennis Capewell, now U.S. Pat. No. 4,663,570.
In the circuits disclosed in the above-noted parent application, gas discharge lamp current is indirectly monitored by monitoring the current through a power transistor switch which applies power in a controlled manner to the lamp ballast, rather than by directly monitoring the lamp current. This has the advantage of isolating the current sensing circuitry from the lamp lead wiring so that miswires at the lamp will not defeat the current measuring loop operation which would lead to the destruction of the power transistor.
A disadvantage of this arrangement occurs when the lamp is operated at very low light output, for example at one percent of the full light output of the lamp. Under this condition, the lamp filament current becomes a significant portion of the total current to the lamp load. Therefore, the current through the transistor switch is no longer an accurate measure of the actual arc power in the lamp. By way of example, in a 40 watt lamp the actual lamp arc power when dimmed to one percent of full light will be less than about one-half watt. The lamp filaments, however, require a relatively constant power which is independent of light output and may consume about one watt each. Thus, the total power to the lamp when dimmed to one percent of full light output is about 21/2 watts but only about 20 percent of this contributes to actual light output. If one were to change the power delivered to the lamp from 21/2 watts to 21/4 watts, the actual lamp arc power will be reduced by 50 percent (from 1/2 watt to 1/4 watt). Thus the light output will change by 50 percent due to a change in the transistor switch current of less than 15 percent.
At higher light levels where the arc power is greater than about four watts (10 percent of the total light output of the 40 watt lamp), the distinction between the current measured in the power transistor as compared to the total power to the lamp is relatively unimportant.
This sensitivity to very small variations in the switch current at low dim levels makes it difficult to maintain accurate dimming control at relatively low dim values, for example less than about 10 percent of the full light output. Accurate low end control, however, is necessary, particularly when several lamps near one another are operated from separate dimmers since their visual appearance at low dim may be very different. Moreover, it is desirable to have accurate low dim control to prevent dimming below safe levels.